The Integrated Forecast System (IFS) code is a parallel MPI application running on multiple tasks, a specified number of which, during its execution, writes output to a single global file at the end of several output time intervals. It can therefore write output multiple times during a given run. With the appropriate choice of parallel writing routine the overhead of writing to disk can be effectively hidden from the computation. We shall show how this is possible with careful use of MPI-IO routines on top of the IBM General Parallel File System (GPFS).